1. Field of the Invention
The present invention relates to a method of manufacturing a contact image sensor for reading an image document or the like, and more particularly to a method of manufacturing a contact image sensor having a structure which is prevented from electrostatic destruction due to triboelectricity produced when an image document or the like moves with respect to the contact image sensor.
2. Description of the Related Art
Image sensors are used to read an image document and convert the image document into an electric signal in image scanners, facsimile machines, or the like. The image sensors that are available at present are roughly classified into two types, i.e., an optical image sensor having a size-reduction optical system including lenses for focusing an image document at a reduced scale onto a CCD (charge-coupled device) to read the image document, and a contact image sensor having a sensor unit composed of a plurality of sensor elements formed according to a thin-film fabrication process on an insulating substrate which has a width greater than the width of an image document to be read, the sensor unit being held in direct contact with the image document without any size-reduction optical system interposed therebetween.
The contact image sensor may be easily reduced in size and cost because it uses no lenses. Since the contact image sensor is fabricated according to a thin-film fabrication process, it can be mass-produced by forming a plurality of image sensor patterns on a single insulating substrate and then cutting off the substrate into a plurality of contact image sensors.
The contact image sensor, however, has a problem in that it can easily be electrostatically charged because the insulating substrate is made of glass or the like. In the fabrication process, the insulating substrate tends to be electrostatically charged, causing sensor elements on the insulating substrate to be electrostatically destroyed. The susceptibility to triboelectric destruction has been responsible for a low yield of contact image sensors.
Japanese laid-open patent publication No. 2-98966 discloses a method of fabricating a contact image sensor in an attempt to solve the above problem. As shown in FIG. 1 of the accompanying drawings., a plurality of individual electrodes 302 of chromium and a short-circuiting pattern 303 are formed on a glass substrate 301. The individual electrodes 302 are composed of pixel electrodes 302a and leading electrodes 302b, the pixel electrodes 302a being arranged in two rows A, B extending in the longitudinal direction of the glass substrate 301. The short-circuiting pattern 303 is positioned between the two rows of the pixel electrodes 302a, and comprises a horizontal pattern 303a extending in the longitudinal direction of the glass substrate 301 and a plurality of vertical patterns 303b extending transversely across the horizontal pattern 303a and connecting respective pairs of the pixel electrodes 302a.
All the pixel electrodes 302a are covered with a photosemiconductor layer 308, on which there are disposed two common electrodes 304 in covering relationship to the pixel electrodes 302a of the respective rows A, B, thus producing two elongate sensor units of sandwiched structure on the glass substrate 301.
Then, a plurality of ICs 306 are mounted on the glass substrate 301, and pads of the ICs 306 and ends of the leading electrodes 302b are interconnected by bonding wires 307. Thereafter, the glass substrate 301 is cut off along the horizontal pattern 303a by a slicing machine having a slicing blade which is wider than the horizontal pattern 303a, breaking the electric connection between the paired individual electrodes 302. Before the ICs 306 and the individual electrodes 302 are connected, the individual electrodes 302 and the common electrodes 304 remain connected to each other with no potential difference developed therebetween. Accordingly, the sensor unit is prevented from being electrostatically destroyed in the fabrication process.
The contact image sensor operates by holding the sensor units against an image document, and scanning the sensor units over the image document to read an image thereon. Since the image document is normally made of paper and hence is insulative, triboelectricity is generated between the glass substrate and the image document. FIG. 2 of the accompanying drawings shows a cross section of the contact image sensor after the glass substrate 301 is cut off along the horizontal pattern 303a. Since the vertical patterns 303b extending from the individual electrodes 302 have ends exposed in a region where the image document moves with respect to the contact image sensor, triboelectricity tends to be discharged to the exposed ends of the vertical patterns 303b, and the sensor unit is likely to be electrostatically broken during usage of the contact image sensor.
One solution would be to position the exposed ends of the vertical patterns 303b outside of the region where the image document moves with respect to the contact image sensor. However, because all the vertical patterns 303b extend from the individual electrodes 302, a very wide wiring pattern area would be needed to position the exposed ends of all the vertical patterns 303b outside of the region where the image document moves with respect to the contact image sensor. Not only signal wires but also a new wire just for short-circuiting the individual electrodes would be required in such a very wide wiring pattern area. Consequently, such an arrangement would result in an increase in the cost owing to an increase in the size of the glass substrate, and would be uneconomical in terms of effective utilization of the area available on the glass substrate.